In the majority opinion, Justice Elena Kagan wrote that the law violated the Eighth Amendment's ban on cruel and unusual punishment.

The AP reports:

"The 5-4 decision is in line with others the court has made, including ruling out the death penalty for juveniles and life without parole for young people whose crimes did not involve killing.

"The decision came in the robbery and murder cases of Evan Miller and Kuntrell Jackson, who were 14 when they were convicted. Miller was convicted of killing a man in Alabama. Jackson was convicted of being an accomplice in an Arkansas robbery that ended in murder."

In their concurring opinion Justice Stephen Breyer and Justice Sonia Sotomayor explain that in previous cases, the court has decided that juveniles deserve special consideration. Quoting a previous decision, the justices write that for one thing, juveniles have a "lack of maturity" and science continues "to show fundamental differences between juvenile and adult minds..."

"For another thing, [2010's Graham v. Florida] recognized that lack of intent normally diminishes the 'moral culpability' that attaches to the crime in question, making those that do not intend to kill 'categorically less deserving of the most serious forms of punishment than are murderers,'" the justices write.

To slap juveniles with a mandatory sentence, which does not allow a judge to take into consideration the circumstances of a murder, therefore violates the Eighth Amendment, the justices said.

Chief Justice John Roberts issued a dissenting opinion saying the court is interpreting the Eighth Amendment as banning the death penalty, which the court does not characterize as "cruel and unusual."

Justice Clarence Thomas also issued a dissenting opinion, wrapping up his reasoning with this scathing paragraph:

"Today's decision invalidates a constitutionally permissible sentencing system based on nothing more than the Court's belief that 'its own sense of morality ... pre-empts that of the people and their representatives." Graham, supra, at ___ (THOMAS, J., dissenting) (slip op., at 29). Because nothing in the Constitution grants the Court the authority it exercises today, I respectfully dissent."